What is Flow Restrictor?

A flow restrictor, as the name suggests, restricts the flow of RO reject water. Flow restrictor maintains high pressure inside the RO membrane (a precondition for RO purification) by creating back pressure on the membrane and also controls the recovery ratio of the purifier.

An RO Flow Restrictor of 550 mL

If flow restrictor is not used then the high-pressure water coming in from the booster pump will flow unrestricted from the reject outlet of the RO membrane. This will result in low pressure inside the RO membrane and higher wastage of water. RO membrane won’t be able to work if the pressure inside the RO membrane is not high.

See the below water flow diagram of an RO water purifier to understand the working of a flow restrictor.

Flow Restrictor in RO Water Purifier

Flow restrictors are generally available in different flow ratings and must be properly sized according to the capacity of the RO membrane.

How to Find a Matching Flow Restrictor?

The ratings that you see on flow restrictors, like FR 200, FR 300, FR 450, indicate the flow rate, in mL per minute (Millilitres Per Minute).

RO membrane capacity is given in GPD (Gallons Per Day).

1 Gallon is approximately equal to 3.78 litres.

Generally, the flow restrictor value should be 4 times the production rating or the capacity of the membrane.

In order to find a matching flow restrictor, first, we need to convert the flow restrictor’s capacity value into GPD.

This is quite simple, and all you need to do is simply multiply the flow restrictor capacity in mL per minute by 0.38 to convert it into GPD.

Example of Flow Restrictor for 800 mL is:

800 x 0.38 = 304 GPD

As we know the flow restrictor rating should be 4 times the RO membrane.

Dividing the above rating by 4 i.e. 304/4, we get 76 GPD. So for an RO membrane of 75-80 GPD, you should use 800 mL flow restrictor.

In India, most technicians are using Flow Restrictor 450 with 75 GPD or 80 GPD RO membrane. But, we recommend a flow restrictor of at least 550 ml.

The recommended flow restrictor sizes are listed in the below table.

Membrane Size (in GPD)

Flow Restrictor (mL per minute)

18

200

25

250

36

250 or 360

50

420 or 525

75

550 to 800

80

800

100

800

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What if I use a Flow Restrictor of a Higher or Lower Capacity?

The RO membrane GPD and flow restrictor capacity must be matched to keep a proper balance between water wastage and purification quality. If they are not matched, this can cause excessive wastage of water or may reduce the life of the RO membrane.

Under Sizing

If you are using a lower capacity flow restrictor for a higher capacity membrane then it will result in early choking of the RO membrane, reduced membrane life, and an increase in TDS value of the purified water.

Over-Sizing

And if you use higher capacity flow restrictor then it will result in higher wastage of water and lower pressure inside the RO membrane. Low pressure inside RO membrane can have an adverse effect on the quality of purification.

When to Change the Flow Restrictor?

We recommend that the flow restrictor should be replaced every time an RO membrane is replaced. Because over time, like RO membrane and filters, flow restrictors can also get choked because of scale build up.

And if the new RO membrane is of a different capacity then also the flow restrictor must be changed according to the capacity of the RO membrane.